Rate constants of electronic energy transfer from aromatic triplets to chromium(III) complexes. Role of the preexponential factor

The variations in the rate constants for the energy-transfer quenching of the lowest excited triplet of anthracene, acridine, and naphthalene by 11 Cr(III) complexes are examined. It is shown that, in contrast to a previous assumption, the low values of kq exhibited by most complexes cannot be attributed to an activation energy term. Consideration of the difficulty in the overlap between the donor orbitals and the acceptor (metal-centered) orbitals suggests that the reason for the low kq values is to be found in low values of the preexponential factor (ken0) of the energy-transfer rate constants. This suggestion is supported by the correlation found between ken0 and the nephelauxetic parameter, β, of the various complexes.